Conventional moisture sensing systems include a Loop Powered Dewpoint Transmitter (LPDT), such as the hyper thin film HTF™ Al2O sensor provided by Xentaur Corporation. The LPDT system allows a user to view dewpoint by monitoring the operation of a sensor constructed as a capacitor having a dielectric that consists of porous aluminum oxide, as well as monitoring the gas that enters pores of the aluminum oxide. The capacitor plates, i.e. electrodes, are an aluminum substrate and a porous gold layer deposited on top of the aluminum oxide, and the electrode having the porous gold layer allows transfer of gases into or out of the aluminum oxide pores. The capacitance due to the aluminum oxide is constant, while the capacitance due to the adsorption of water on the surface of the aluminum oxide dielectric will vary according to the water vapor content and pressure. Since the dielectric constant of adsorbed water is orders of magnitude larger than the dielectric constant of any gas that might be adsorbed and measured, the quantity of water vapor present in the pores resulting in water adsorption on the dielectric will change the capacitance of the sensor to a much greater extent than other variables.
However, the use of an LPDT sensor will often cause pore volume to change due to contaminant clogging, residual oxidation, metal migration etc. Thus, re-calibration of the sensor is needed.